Method for the modification of polyolefins

ABSTRACT

A process for obtaining a polyolefin composition endowed with improved adhesion performance, especially paintability, comprising premixing a high-flow polyolefin resin with one or more grafting monomers and initiator, possibly also one or more comonomers, feeding the obtained premix and a low-flow polyolefin resin into a mixing device, and gradually heating the components to above their melting points while being subjected to mixing forces. The process may be carried out in an extruder, and polyether amine may optionally be introduced into the extruding mass downstream of the feeding point.

FIELD OF THE INVENTION

The present invention relates to a process for the modification ofpolyolefins to give them improved adhesion performances.

RELATED ART

Polyolefins are known to present poor adhesion to paints, glues andinks. To improve their adhesion properties they need to be chemicallymodified with polar groups. This could be achieved by blending oralloying the polyolefin resin with engineering polymers, by thefunctionalization of the polyolefin with specific monomers, or byapplying post-treatment methods, such as the use of oxygen plasma andflaming. Polyolefins being modified by graft reactions performed in meltprocessing equipment, such as single or multiple screw extruders,include well known technologies widely reported in the technicalliterature.

U.S. Pat. No. 5,001,197 discloses a process for functionalization andpreferably maleation of a polypropylene by the use of a selected classof peroxides that will not significantly decreased the molecular weightof the polypropylene.

EP 0 634 424 discloses a compound comprised of polypropylene and thereaction product of a maleated polypropylene and polyether amine. Aprocess for producing the polymer compound involves two steps, firstpreparation of the maleated polypropylene, then reacting this maleatedpolypropylene with the polyether amine by melting the components in thepresence of a polypropylene in a customary mixing apparatus.

U.S. Pat. No. 5,281,670 discloses a blend comprising polybutylene andfunctionalized polyolefins having an improved adhesion to metals andpolar polymers. The blend can be made by any conventional blendingprocess, which in general include the steps of tumbling the polybutyleneand functionalized polyolefin in a mixer, feeding the mixture into amelt mixing device, such as a Brabender mixer or continuous single ortwin screw extruder.

WO 96/06872 discloses a method of grafting a polypropylene resin with anallyl epoxy compound in the presence of styrene. The polypropylene mixedwith the allyl epoxy compound, styrene and a peroxide initiator isheated to a suitable reaction temperature at conditions of continuousmixing, preferably in an extruder, until a desired degree of graftinghas been obtained. The disclosures of this publication is includedherein by reference.

U.S. Pat. No. 5,424,367 discloses an extruder having multiple reactionzones. Each reaction zone is provided with means for introducingreagents, mixing the reagents with polymer and removal of bi/co-productsor unreacted reagents. Each reaction zone is followed by a purificationzone where volatile products are vented off. This extruder is used ingrafting a polyolefin with maleic anhydride, followed by imidization ofthe grafted anhydride with a primary amine in the second reaction zone.

The prior art grafting methods used to modify polyolefins will notresult in a sufficient level of polar molecules in the surface layer ofthe final article to obtain an acceptable paintability. Moreover, theoverall desired mechanical properties will become significantly reduced.Blending methods normally involve several compounding steps and the costof the resulting material will hardly be justified. The post-treatmentmethods mentioned above, which are used to increase directly thepolarity of the surface to be painted, are normally very time-consuming,and often high investments in special equipment will be needed.Therefore, it still exists a need for polyolefin resins that areavailable at an acceptable cost and which can be manufactured into finalarticles having an inherent paintability.

It has now surprisingly been found that an extrusion process may be usedto obtain a modified polyolefin blend comprising polypropylene resinsthat are selectively functionalized with polar molecules. In a finalarticle obtained according to the present invention the polar moleculeswill become localized in the surface layer of the article and thus makeit paintable.

SUMMARY OF THE INVENTION

The present invention provides a process for obtaining a polyolefincomposition endowed with improved adhesion performance, comprising thesteps of premixing a high-flow polyolefin resin with one or moregrafting monomers and initiator, optionally also one or more comonomers,feeding the obtained premix and a low-flow polyolefin resin into amixing device, and gradually heating the components to above theirmelting points while being subjected to mixing forces.

The present invention also provides process for obtaining a polyolefincomposition endowed with improved adhesion performance, comprising thesteps of premixing a high-flow polypropylene polymer with one or moregrafting monomers and initiator, optionally also one or more comonomers,feeding the obtained premix and a low-flow polyolefin resin into anextruder or a series of extruders provided with mixing elements,gradually heating the components to above their melting points whilebeing subjected to mixing forces, and introducing at least one polyetheramine into the extruder through a second inlet feed opening, downstreamof the first inlet opening, localized at a point where at least a mainpart of the already introduced components are in a molten state.

DETAILED DESCRIPTION OF THE INVENTION

To obtain a polyolefin based article that can be painted, at least thesurface layer of the article must contain polar groups that make thesurface material compatible with the applied paint film material. By thepresent invention the polyolefin material is blended with a polarcomponent that will migrate to the surface of the article andselectively provide the surface layer with polar groups which make thearticle paintable. To maintain the favourable properties of polyolefinsboth components should be based on polyolefin resins, preferablypolypropylene based resins. Consequently, the polyolefin resin to begrafted is selected from polypropylenes having relatively short chainlengths. However, such polypropylene grades will not alone have anacceptable range of properties and therefore must be-supported byanother polyolefin resin. Thus, in a final article the graftedpolypropylene component will provide the paintability, and the otherpolyolefin components will provide the required mechanical properties.

According to the present invention it is provided a process forsequentially grafting a high-flow polypropylene resin and then blendingthis grafted resin with a low-flow polyolefin resin. The selectedpolypropylene and polyolefin grades must be matched with each other insuch a way that the one to be grafted will melt first and react with thegrafting monomers before any substantial part of the low-flowpolypropylene component has melted. This is achieved by the process ofthe present invention.

In this application the term “high-flow resin” is used to describe aresin that melts relatively easy and has a lower melt viscosity, whilethe term “low-flow resin” is used to describe a resin that has a highermelt viscosity.

Suitable high-flow polypropylene resins to be used by the presentprocess comprise all polypropylene homopolymers and copolymers that canbe grafted with reactive, functionalizing agents. Particularly preferredpolypropylenes are polypropylene homopolymers, and copolymers ofpropylene with ethylene, containing up to 30% of ethylene. Thesepolypropylenes should preferably have a weight average molecular weight(MW) of 5000 to 500.000, preferably from 10.000 to 200.000. The meltflow index (MFI) should be in the range from 0.1 to 1000 g/10 min,preferably from 20 to 1000 g/10 min, more preferably from 20 to 100 g/10min, determined according to the method of ASTM D 1238 (230° C. and 2.16kg load). The polypropylene resins may be used as pellets or as apowder, preferably as a powder. The powder may be milled granules orcorresponding material, or it may be the polymer substance as obtainedfrom a polymerisation reactor, stabilized or non-stabilized.

Suitable low-flow polyolefin resins to be used by the present processin-principle comprise all polypropylene homopolymers and copolymershaving a suitable melt flow index that is lower than the melt flow indexof the used high-flow resin. The melt flow index may be in the rangefrom 0.01 to 80 g/10 min, preferably from 0.1 to 30 g/10 min, determinedaccording to the method of ASTM D 1238 (230° C. and 2.16 kg load).Preferred polyolefins are copolymers of propylene with ethylene, havinga high ethylene content. Particularly suitable polyolefins are reactormade thermoplastic ethylene-propylene copolymers (R-TPO) containing upto 50% of ethylene. Such polyolefins should preferably have a weightaverage molecular weight (MW) in the range from 150.000 to 500.000. Inthe process of the present invention this polyolefin is convenientlyused as pellets.

To initiate the grafting process a number of free radical generatingcompounds may be used, for example peroxide compounds, or initiation byradiation. Organic peroxides that are particularly useful asradical-generating compounds in the grafting process are those whichhave a decomposition temperature lower than the processing temperatureof the high-flow polypropylene resin to be grafted. Suitable peroxideinitiators may be selected from the group comprising t-butyl-peroxybenzoate, t-butyl-peroxy-n-butyl fumarate, azo-bis-butyronitril,bis-tert-butyl peroxy-isopropyl benzene, and dicumyl peroxide.

The functionalizing agent employed in the process of this invention maybe any of the unsaturated monomers conventionally used to functionalizepolyolefins. Such functionalizing agents include olefinicallyunsaturated acids, for example monocarboxylic acids, such as acrylic andmethacrylic acid, dicarboxylic acids, such as fumaric, maleic anditaconic acids; dicarboxylic anhydrides, such as maleic anhydride, andthe corresponding tert.-butyl esters thereof. A preferredfunctionalizing agent for use in the process of the present invention ismaleic anhydride (MAH).

Other compounds suitable for being grafted onto the polymer chains maybe selected from the group comprising sulfo- or sulfonyl-containingmonomers; oxazolinyl-containing monomers; and epoxy-containing monomers,such as allyl glycidyl ether, glycidyl acrylate or glycidyl(meth)acrylate. Particularly useful epoxy-containing monomers are thosehaving the formula:

in which R is H or a C₁₋₄ alkyl; and R₁ is —(CH₂)_(n)—,—C(O)O—(CH₂)_(n)—, or —(CH₂)_(n)—O—, and n is an integer of 1 to 4.Preferably R is H or CH₃, more preferably CH₃, while R₁ is preferably—C(O)O—(CH₂)_(n)—, making glycidyl methacrylate a preferred compound.

Also styrenic compounds may be grafted onto the polymer chains as acomonomer. Suitable styrenic compounds are those having the formula:

in which R₂ is H, OH, CH₃ or allyl. Preferably R₂ is H, making styrenethe most preferred styrenic compound.

The required grafting conditions may conveniently be obtained in anumber of mixing devices known within the field of plastic processing. Apreferred mixing device is an extruder or a series of extruders ofsuitable size and design. A twin screw extruder having co-rotatingintermeshing screws or a single screw extruder may be used alone or inany combination, such as in tandem extrusions. The extruder may beprovided with a vacuum venting at the extruder outlet. The graftingconditions have to be adapted to the type of extruder used, as will bewell known to a skilled person. A prerequisite for achieving a desireddegree of grafting is to obtain an intimate mixing of the polypropylene,the initiator and the grafting monomers at a temperature that is higherthan both the melting point of the polymer and the decompositiontemperature of the peroxide. It is also important to have a sufficientresidence time to secure the completion of the grafting reactions. Thetemperature inside the extruder is within the range from 90 to 300° C.,preferably within the range of 100 to 250° C.

The components may be fed to the mixing machine in any practical way.Optionally, they may be premixed by the use of any conventional mixer,or by any mixing method known in the art. All components are fed fromthe same feed hopper into the extruder.

To obtain an optimal degree of grafting, 100 parts by weight of thehigh-flow polypropylene resin, preferably in the form of a powder, isadmixed with up to 10 parts by weight of the peroxide compound and up to10 parts by weight of the grafting monomer. Obviously, the actualamounts of the peroxide and monomer compounds will depend on theparticular types of compounds used, and the invention is not restrictedto specific amounts of the grafting compounds. This mixture of high-flowpolypropylene, peroxide and monomer is then blended for a sufficientperiod of time to obtain a proper distribution and absorption of thereactive components into the powder. This mixture is then fed into theextruder together with a low-flow polyolefin material, preferably beingin the form of pellets. Inside the extruder the high-flow polypropylenepowder will have melted completely before a substantial part of thelow-flow polyolefin pellets have melted. At the used temperature settingthe peroxide will decompose and initiate the grafting reactions. Thegrafting monomers will react mainly with the easy flowing molten polymerchains of the high-flow polypropylene resin, which will be selectivelygrafted. As the low-flow polyolefin resin melts it will be blended withthe already grafted resin. The final homogeneous melt is ultimatelyextruded, for instance as strands which may be pelletised in aconventional manner.

A composition obtained by the present process may contain an amount ofgrafted monomer up to 15%, preferably up to 10%, more preferably from 2to 5% by weight of the starting, neat polyolefin resin.

To further improve the polar properties of the final polymer compositionthe monomers grafted onto the high-flow polypropylene may in a furtherstep be reacted for example with a suitable amine compound, such as amono- or diamine, preferably a diamine. Conventionally used amines arepolyether amines. Suitable polyether blocks in the polyether amine maybe selected from the group comprising polyethylene glycol, polypropyleneglycol, copolymers thereof, poly(1,2-butylene-glycol) andpoly(tetramethyleneglycol). Preferred polyether amines includemonoamines, diamines and triamines, having a molecular weight from 150to 12000 g/mol. More preferred are polyether diamines having a molecularweight of about 2000 g/mol.

The polyether amine is reacted with the grafted monomers in anintegrated step of the very same extrusion process. This is achieved byintroducing a polyether amine into the extruder through a second feedinlet opening situated downstream of the first inlet opening. Thissecond inlet opening must be positioned in a distance from the firstinlet opening where at least a substantial part of the polypropylene hasbeen grafted with the functionalizing monomer. Optionally, when a tandemextruder line is used this second component may be introduced into thesecond, tandem extruder. The polyether amine will thereby become mixedinto the melt and react with the chains grafted onto the polypropylenebackbone. The obtained blend may then be extruded as explained above.

The composition of the invention is especially well suited for use as anengineering plastic resin in articles that are to be painted, printed ordyed. In such articles a substantial part of the high-flow graftedpolypropylene component of the composition will migrate to the surfaceof the article. As a result, the surface will become enriched with polarmolecules, and this will highly improve the adhesion between the surfaceof the article and applied substrates containing polar polymers, such aspaints. In particular, this is important in painted articles, such asautomotive interior and exterior parts. Articles made from thecompositions of the present invention will be endowed with improvedprintability, often important within fields such as household appliancesand packaging. Improved dyeability is important in articles such asfibers.

Even if the present invention is described with reference to polyolefinresins, and in particular polypropylene resins, the concept of thisinvention can easily be applied with a wide range of polymericmaterials, such as polypropylene homopolymers; copolymers of propylenewith ethylene, containing from 0.1 to 25% by weight of ethylene; blendsof polypropylene with polyethylenes such as high density polyethylene(HDPE), low density polyethylene (LDPE), linear low density polyethylene(LLDPE); ethylene propylene rubbers (EPR); alloys of polypropylene withanother thermoplastic polymer selected from the group comprisingpolyamides (PA), polyethylene terephthalates (PET), polybutyleneterephthalates (PBT), polycarbonates (PC) and polystyrenes (PS); andethylene acrylic copolymer such as EMA, EBA, and EEA. Polypropyleneblends or alloys may contain the second polymer component in an amountfrom 0.1 to 40% by weight based on the weight of the polymercomposition.

The word “comprising” and other forms of the word “comprising” used inthis description and in the claims does not limit the claimed inventionto exclude any variations or additions which are obvious to the personskilled in the art and which do not have a material effect upon theinvention.

The invention is illustrated with the following working examples showingin more details specific features and preferred embodiments of theinvention. However, the invention is not restricted to theseembodiments, and variations and deviations therefrom will be obvious toa person skilled in the art.

EXAMPLES EQUIPMENT

The extruder used in the examples was a Clextral BC 21 twin screwextruder having a screw diameter of D=25 mm and a barrel length of L=44D.

COMPONENTS

The following components were used in the examples:

High-flow polypronylene resin, a PP homopolymer having a MFI of 37 g/10min (230° C./2.16 kg), commercially available under the trade name of“Borealis HH 420 J” from Borealis AS.

Low-flow polvolefin resin, a reactor produced thermoplasticethylene-propylene copolymer (R-TPO) having a MFI of 8 g/10 min (230°C./2.16 kg), commercially available under the trade name of “Borealis ED050 T” from Borealis AS.

Polyether diamine (PEA), commercially available under the trade name of“Jeffamine XTJ 418” from Huntsman Corp.

Peroxide, t-butyl peroxy benzoate, commercially available as a 98%solution under the trade name of “Trigonox C” from Akzo Nobel.

Maleic anhydride (MAH) was of a common commercial grade.

Test Methods

The melt flow index was determined according to the method of ASTM 1238.

The materials obtained in the examples were injection moulded intosheets having a thickness of about 2 mm and stored at ambient conditionsfor 2 days. The sheets were then subjected to the Gitterschnitt paintadhesion test according to the test method of DIN 53151. The paintadhesion tests were performed with solvent-borne polyurethane “2K PUR”paints. The adhesion was evaluated and classified according to100%=excellent adhesion, 0%=no adhesion.

In the following examples all quantities of constituents are given inparts by weight based on 100 parts by weight of the final composition.Amounts used and results obtained are presented in Table 1.

Example 1

A high-flow polypropylene resin taken directly from the reactor as apowder was used. A quantity of 86 parts of this polypropylene resin wasmixed with 2.5 parts of maleic anhydride and 1.5 parts of peroxide, andblended for 1 hour. The obtained mixture and 10 parts of R-TPO polymerwere fed together to the extruder specified above. The temperature ofthe extruder was set at 200° C., and the screw speed was adjusted to 150rpm, which resulted in an output of 3 kg/h. This blend was extruded asstrands which were cooled by water and chopped into granules in aconventional manner.

Example 2

The procedure of Example 1 was followed, except that 84 parts of thepolypropylene resin, 4.0 parts of maleic anhydride, 2 parts of theperoxide and 10 parts of the R-TPO polymer were used.

Example 3

The procedure of Example 1 was followed, except that 64 parts of thepolypropylene resin, 4 parts of maleic anhydride, 2 parts of theperoxide and 30 parts of R-TPO were used.

Example 4

The procedure of Example 1 was followed, except that 54 parts of thepolypropylene resin, 4 parts of maleic anhydride, 2 parts of theperoxide and 40 parts of R-TPO were used.

Example 5

The procedure of Example 1 was followed, except that 20 parts of thepolypropylene resin, 4 parts of maleic anhydride, 2 parts of theperoxide and 74 parts of R-TPO were used.

Example 6 (comparative)

The high-flow polypropylene resin used in Example 1 was extruded aloneat the conditions specified above.

Example 7 (comparative)

The R-TPO resin used in Example 1 was extruded alone at the conditionsspecified above.

Example 8 (comparative)

The procedure of Example 3 was followed, except that the maleicanhydride monomer and peroxide were mixed and absorbed onto R-TPOpellets instead of PP powder.

Example 9

A high-flow polypropylene resin taken directly from the reactor as apowder was used. A quantity of 84 parts this polypropylene resin wasmixed with 4.0 parts of maleic anhydride and 1.5 parts of peroxide, andblended for 1 hour. The obtained mixture and 10 parts of R-TPO polymerwere fed together to the above specified extruder. In a position 20 Ddownstream of the feed inlet opening of the extruder, the polyetherdiamine was introduced into the extruder in a quantity of 3.5 parts perhundred parts (pph) of the combined quantities of PP and R-TPO. Thetemperature profile of the extruder was set at 200° C., and the screwspeed was adjusted to 150 rpm, which resulted in an output of 3 kg/h.This blend was extruded as strands which were cooled by water andchopped into granules in a conventional manner.

Example 10

The procedure of Example 9 was followed, except that 20 parts of thepolypropylene resin, 4 parts of maleic anhydride, 2 parts of theperoxide, 74 parts of R-TPO and 5.0 pph of polyether diamine were used.

Example 11 (comparative)

The composition obtained in Example 5 was subsequently in a separatestep mixed with 5 pph of the polyether diamine and extruded in aconventional manner at the extrusion conditions used in Example 1.

This example represents the traditional two step method of producing amodified PP composition.

The testing parameters and the obtained results are summarized inTable 1. The polypropylene based compositions obtained by the process ofthe present invention (Examples 1 to 5 and 9 to 10) have excellentadhesion properties and perform equally well to the composition obtainedby the prior art process (Example 11). The polyolefin grades notmodified (comparative examples 6 and 7) have no adhesion to paint atall.

The results show that the present process allows specificfunctionalization of a high-flow polypropylene component andcorresponding improvement of the adhesion properties of the obtainedcomposition. Moreover, coupling of the grafted monomers with e.g.polyether diamines during the same one step extrusion process is also anoption.

TABLE 1 Paint Exam- PP ho TPO Peroxide MAH PEA adhe- ple (parts) (parts)(parts) (parts) pph sion, %  1 86 10 1.5 2.5 none  80-100  2 84 10 2.04.0 none 100  3 64 30 2.0 4.0 none 100  4 54 40 2.0 4.0 none 100  5 2074 2.0 4.0 none  80-100  6 100  none  0 Comp.  7 100  none  0 Comp.  864 30 2.0 4.0 none 10-30 Comp.^(a)  9 84 10 2.0 4.0 3.5 100 10 20 74 2.04.0 5.0 100 11 20 74 2.0 4.0 5.0 100 Comp.^(b) ^(a)MAH mixed with TPOinstead of PP ^(b)Two-step process

What is claimed is:
 1. A process for obtaining a polyolefin compositionendowed with improved adhesion performance, comprising the steps of: i)premising a high-flow polyolefin resin with one or more graftingmonomers and initiator, optionally also one or more comonomers, ii)feeding the obtained premix and low-flow polyolefin resin into a mixingdevice, and gradually heating the components to above their meltingpoints while being subjected to mixing forces.
 2. The process of claim1, wherein the polyolefin resins are polypropylene based resins.
 3. Theprocess of claim 1, wherein the high-flow polypropylene resin has a MFIof 0,1 to 1000 g/10 min.
 4. The process of claim 1, wherein the low-flowpolypropylene resin has a MFI of 0,01 to 80 g/10 min.
 5. The process ofclaim 1, wherein the high-flow polypropylene is in the form of a powderand that the low-flow polypropylene is in the form of pellets.
 6. Theprocess of claim 1, wherein the grafting monomer is selected from thegroup comprising acrylic and methacrylic acids; fumaric, maleic anditaconic acids; maleic anhydride and the corresponding tert.-butylesters thereof.
 7. The process of claim 1, wherein the initiator isselected from the group comprising t-buytl-peroxy benzoate,t-butyl-peroxy-n-butyl fumarate, azo-bis-butyronitril, bis-tert-butylperoxy-isopropyl benzene, and dicumyl peroxide.
 8. The process of claim1, wherein an extruder is used as the mixing device.
 9. A process forobtaining a polyolefin composition endowed with improved adhesionperformance, comprising the steps of: I) premixing a high-flowpolyolefin resin with one or more grafting monomers and initiator,optionally also one or more comonomers, ii) feeding the obtained premixand a low-flow polyolefin resin into an extruder or a series ofextruders provided with mixing elements, gradually heating thecomponents to above their melting points while being subjected to mixingforces, and iii) introducing at least one polyether amine into theextruder through a second inlet feed opening, downstream of the firstinlet opening, localized at a point where at least a main part of thealready introduced components are in a molten state.
 10. The process ofclaim 9, wherein the grafting monomer is maleic anhydride.
 11. Theprocess of claim 9, wherein the polyether amine is a polyether diamine.12. The process of claim 9, wherein the grafting monomer is styrene. 13.The process of claim 2, wherein the high-flow polypropylene resin has aMFI of 0,1 to 1000 g/10 min.
 14. The process of claim 2, wherein thelow-flow polypropylene resin has a MFI of 0,01 to 80 g/10 min.
 15. Theprocess of claim 3, wherein the low-flow polypropylene resin has a MFIof 0,01 to 80 g/10 min.
 16. The process of claim 2, wherein thehigh-flow polypropylene is in the form of a powder and that the low-flowpolypropylene is in the form of pellets.
 17. The process of claim 3,wherein the high-flow polypropylene is in the form of a powder and thatthe low-flow polypropylene is in the form of pellets.
 18. The process ofclaim 2, wherein the grafting monomer is selected from the groupcomprising acrylic and methacrylic acids; fumaric, maleic and itaconicacids; maleic anhydride and the corresponding tert.-butyl estersthereof.
 19. The process of claim 2, wherein the initiator is selectedfrom the group comprising t-buytl-peroxy benzoate,t-butyl-peroxy-n-butyl fumarate, azo-bis-butyronitril, bis-tert-butylperoxy-isopropyl benzene, and dicumyl peroxide.
 20. The process of claim2, wherein an extruder is used as the mixing device.